Conventional laser light sources include a Fabry-Perot laser light source, which uses a Fabry-Perot resonator, and a distributed feedback (DFB) laser light source, which uses a diffraction grating. These types of laser light sources produce an oscillation of laser light by amplifying light of a predetermined wavelength through resonation or diffraction.
In recent years, new types of laser light sources using a photonic crystal have been developed. A photonic crystal consists of a dielectric body material in which an artificial periodic structure is created. Usually, the periodic structure is created by providing the body material with a periodic arrangement of areas whose refractive index differs from that of the body material (this area is called the “modified refractive index area” hereinafter”). The periodic structure causes a Bragg diffraction within the crystal and produces an oscillation of laser light by amplifying light of a predetermined wavelength. Patent Document 1 discloses a two-dimensional photonic crystal surface-emitting laser light source in which an active layer containing a luminescent material is located between a pair of sheet electrodes and a two-dimensional photonic crystal is provided in the vicinity of that layer.
FIG. 1 shows the details of the structure of this conventional two-dimensional photonic crystal surface-emitting laser light source. This device includes a lower cladding layer (substrate) 11 covered with a spacer layer 161, on which an active layer 12 is located. The active layer 12 is covered with another spacer layer 162, on which a two-dimensional photonic crystal 13 is provided. The active layer 12 is made of a material that will emit light when an electric current flows into it. An example of such a material is a multi-quantum well (MQW) consisting of indium gallium arsenide (InGaAs)/gallium arsenide (GaAs). The two-dimensional photonic crystal 13 consists of a plate member having cylindrical holes 14 periodically arranged in a square lattice pattern. On this two-dimensional photonic crystal 13 are a spacer layer 163, an upper cladding layer 17 and a contact layer 18 laminated in this order. An upper electrode 191 is provided on the contact layer 18 and a lower electrode 192 beneath the lower cladding layer 11. The lower electrode 192 entirely covers the lower surface of the lower cladding layer 11, whereas the upper electrode 18 covers only a central portion of the upper surface of the contact layer 18. It should be noted that FIG. 1 has the two-dimensional photonic crystal 13 intentionally separated from the spacer layer 163 so that the structure of the two-dimensional photonic crystal 13 can be viewed. The two components should be tightly joined together when the device is actually created.
When the laser light produced by this conventional two-dimensional photonic crystal surface-emitting laser light source is observed at a point sufficiently distant from the contact layer 18, it will be found that the main beam is accompanied by weaker beams. These beams are generally called the side lobes. One of the reasons for the emergence of these side lobes is the presence of the upper electrode 191. As shown in FIG. 2, when the laser light is emitted from the surface of the contact layer 18, the upper electrode 191 blocks a portion of the two-dimensional emission. In this situation, rays of light emitted from the areas on both sides of the upper electrode 191 can constructively interfere at points separate from the central beam. Thus, the side lobes 22 are created at the points separate from the main peak 21.
Using an electrode made of a transparent material will most likely prevent the creation of such side lobes. However, the material must satisfy another condition; it must allow electric charges to be efficiently introduced into the active layer. Unfortunately, no materials currently available have these two properties: good transparency and high efficiency for the injection of electric charges.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-332351 (Paragraphs [0037]-[0056], FIG. 1)